1. Field of the Invention
The present invention relates to a photoelectric conversion device, and particularly to a photoelectric conversion device making use of the avalanche multiplication.
2. Related Background Art
Typically, photoelectric conversion devices are required to have a high signal-to-noise ratio for the photoelectric conversion characteristics. In particular, a photoelectric conversion device which uses an avalanche photodiode (hereinafter abbreviated as APD) operated with the avalanche effect in the light receiving portion is expected as it can meet this requirement, and has been developed vigorously in recent years.
Conventionally, generally widespread APDs draw the avalanche effect by applying a strong electric field thereto, in which excessive multiplication noise may occur due to fluctuations contained in the multiplication process, thereby decreasing the signal-to-noise ratio.
In the light of this respect, for example, F. Capasso et al. have proposed a low noise APD applicable to the optical communication system which is fabricated by using single crystal compound semiconductor belonging mainly to the III-V group by molecular beam epitaxy (MBE), in Japanese Patent Application Laid-Open No. 58-157179 and IEEE Electron Device Letters EDL 3rd edition (1982), pp. 71 to 73.
The schematic structural views of a conventional APD which has been proposed therein are illustrated in FIGS. 1 to 3.
FIG. 1 is a cross-sectional structural view of a conventional APD. The I-type band gap graded semiconductor layers 201, 203, 205, 207 and 209, consisting of five layers, acting as the carrier multiplication layer are sandwiched between a P-type semiconductor layer 211 and an N-type semiconductor layer 215, with an electrode 213 being in ohmic contact with the P-type semiconductor layer 211 and an electrode 214 being in ohmic contact with the N-type semiconductor layer 215.
FIG. 2 is an energy band structural diagram when the conventional APD is placed in the operating condition by applying a strong electric field thereto. Herein, since the energy discontinuity of the hetero junctions 202, 204, 206 at which the band gap will steeply step back promotes the ionization, ionization selectively takes place in the neighborhood of the step backs, thereby multiplying carriers.
Such a structure is adopted because it can reduce fluctuation on the sites where ionization takes place, and thus reduce fluctuation contained in the multiplication process. Accordingly, it is possible to realize a low noise APD, which is applicable to the optical communication system, with less excessive noises and an improved signal-to-noise.
However, the conventional APD as above described is effective as a discrete light receiving element for the optical communication which can operate by the application of a strong electric field, but if the conventional APD is used more widely in an application range including photoelectric conversion devices for use with a video camera, a scanner, etc., for performing storage operation, the following problems have often occured.
(1) As the conventional APD is made of a compound semiconductor belonging to the III-V group and the II-VI group as its constituting material, it has some problems with the industrial material concerning the toxicity or price of material.
(2) As in forming a single crystal compound semiconductor as the constituting material thereof, film formation is necessary to be made at high temperatures (about 500.degree. C. or higher) using a ultra-high vacuum equipment, it is difficult to apply it to the photoelectric conversion device of large area, and impossible to laminate it on a semiconductor substrate on which a signal processing circuit is already formed, resulting in the limited application range.
(3) In order to realize a low noise APD, it is necessary to raise the ionization rate at step back hetero junctions, for which a material in which either one of valence band and conduction band is only large on the energy discontinuity at the step back junctions is required, but there are only a limited number of materials meeting such a requirement in the crystal compound semiconductor. Furthermore, in order to realize a much lower noise APD with thermally arising dark current constituting a factor of noise suppressed, it is necessary to meet the above-cited requirement as well by using a material having a large minimum forbidden band width (desirably greater than 1.0 eV), but there is no material meeting such requirement in the crystal compound semiconductor.
(4) Furthermore, when performing the storage operation, the electric field to be applied to APD will decrease with increasing carrier storage amount, so that spike and notch may arise at the step back hetero junctions of the carrier multiplication layer constituted of the I-type semiconductor, as shown in FIG. 3. Therefore, the effective band discontinuity at the step back hetero junctions will degrade, causing a decrease in ionization rate and a discontinuity in energy band in the direction of impeding the carrier transit, thereby resulting in smaller multiplication factor, proper linearity in incident light quantity to output, and lower response speed.